Here's a fic that fits your second request.

"Don't You Quit On Me" by J3 (CaseMatthews). Pairing is Sam/Dean/Cas

Less than one percent of the whole world's population is made up of Omegas, and most people regard them as precious things to be protected and cared for at all cost; but not Alastair. And if anyone knows that, it's Dean.

Dean was taken from them when he was just four years old, Sam's been looking for him for the last twelve years, building a pack, mating with a beautiful beta in that time. Sam knows what taking on an Omega means and he knows perfectly well that Dean's his brother...but what can he do? He needs Dean by his side and if this is the only way, so be it. But Dean's head's all messed up, he doesn't remember his old life, he doesn't remember Sam...should Sam tell him? Should Dean know, after everything he's been forced to live through, that his own Alpha is his own big brother?

The fic is on AO3, but I didn't want to get marked as spam, so I just copied and pasted the back part of the URL here. Just add it to the ao3 URL when you're copying and pasting
/works/1765627/chapters/3777058